The present invention relates to a beam reshaping device, and an optical head device and an optical disk system using the beam reshaping device. The present invention is typically applicable to an optical disk system and an optical head device for recording or reproducing information on or from a phase change type optical disk. In particular, the present invention relates to a beam reshaping device including first and second beam converting means having beam converging powers different in two orthogonal directions wherein the first and second beam converting means are arranged such that conjugate points correspond to each other in the two orthogonal directions, thereby reshaping the sectional form of a laser beam while effectively preventing occurrence of astigmatism of the laser beam.
A known optical disk system is configured to record or reproduce information on or from an optical disk by converging a laser beam emitted from a laser diode on the optical disk and receiving a return beam reflected from the optical disk.
In such an optical disk system, a laser beam emitted from a laser diode is converted into a collimated beam by a collimator lens and is converged on an optical disk, and an anamorphic prism is interposed in the optical path as needed to reshape an elliptic form in cross-section of the laser beam into a circular form. The technique for reshaping the sectional form of a laser beam has been disclosed in, for example, U.S. Pat. Nos. 5,181,224 and U.S. Pat. No. 5,081,639.
On the other hand, to enhance the recording density, the optical disk system may be required to correct the spherical aberration of a laser beam caused by a variation in thickness of a light transmission layer of an optical disk or the accuracy of an optical part.
In the case of reshaping the sectional form of a laser beam emitted from a light source by using the anamorphic prism, however, there arises a problem associated with occurrence of astigmatism of the laser beam. If such astigmatism occurs, it becomes difficult for the optical disk system to simply correct the spherical aberration of the laser beam. Further, in the case of using a semiconductor laser as the light source, since a laser beam emitted from the semiconductor laser contains astigmatism, it is required to correct such astigmatism.
An object of the present invention is to provide a beam reshaping device capable of reshaping the sectional form of a laser beam while effectively preventing occurrence of astigmatism of the laser beam, and an optical disk system and an optical head device using the beam reshaping device.
To achieve the above object, according to the present invention, there is provided an optical disk system for recording or reproducing information on or from an optical disk by irradiating the optical disk with a laser beam, the optical disk system including: a laser light source for emitting a laser beam; a beam reshaping means for reshaping the sectional form of the laser beam emitted from the laser light source; a converging means for converging the laser beam reshaped by the beam reshaping means on the optical disk. The beam reshaping means includes: a first beam converting means having beam converging powers different in a direction along a first virtual line perpendicular to an optical axis and in a direction along a second virtual line perpendicular to the optical axis and the first virtual line, the means allowing the laser beam having been made incident on the means to pass therethrough; and a second beam converting means having beam converging powers different in the direction along the first virtual line and in the direction along the second virtual line, the means allowing the laser beam having passed through the first beam converting means to pass therethrough; wherein the first beam converting means and the second beam converting means are arranged such that conjugate points correspond to each other in the direction along the first virtual line and in the direction along the second virtual line.
In the above first and second beam converting means having beam converting powers different in two direction, conjugate points are allowed to correspond to each other in the two directions by suitably selecting the powers, the distance between the first and second beam converting means, and the like. As a result, it is possible to converge the laser beam emerged from one image formation point on the corresponding objective point without occurrence of astigmatism of the laser beam, and to variously reshape the sectional form of the laser beam by suitably setting the extension degrees of the laser beam in the two directions by changing the powers and the like.
According to the present invention, since the first and second beam converting means having beam converging powers different in two orthogonal directions are arranged such that conjugate points correspond to each other in the two orthogonal directions, it is possible to reshape the sectional form of a laser beam while effectively preventing occurrence of astigmatism upon beam reshaping, and if the laser beam emitted from a light source contains astigmatism, it is possible to suitably correct the astigmatism contained in the laser beam.